Indium is an essential rare metal for the production of ITO (indium-tin oxide) constituting a transparent electrode film material used for an LCD (a liquid crystal panel display). The reserves of indium as a natural resource are unevenly distributed and located in China. Under circumstances in which indium as a natural resource is substantially monopolized by China, Japan is a major indium-consuming country where the demand for the ground metal accounts for over 80% of the global demand (yearly).
In our country, most of indium is used for the production of ITO for an LCD, and, due to upsizing in an LCD and the rapid expansion of the market, there has been a sharp increase in demand therefor. In recent years, with regard to scrap generated in the production process such as a used ITO target material and a deposit on a tool for sputtering, the recycling has proceeded. However, it is thought that indium recovery has not been performed against home electric appliances including a used LCD and a failed panel generated in an LCD factory, and recovering indium from an indium-containing product such as the used LCD and the failed LCD is becoming an important issue. In particular, since an LCD television and the like are added to the list of objective items for Home Appliance Recycling Law, establishing a technology for recycling indium from a used LCD and the like is an important issue from the aspects of securing a stable supply of resources such as rare metals and attaining the volume reduction and detoxification of inorganic waste.
There have been some reports on the study relating to indium recovery from a used LCD product. An adsorption/desorption method using an ion exchange resin against an LCD leachate leaching with hydrochloric acid has been proposed by an LCD manufacturer (NPD 1: T. Honma and T. Muratani: Sharp Gihoh 92 (2005) 17-22, NPD 2: M. Tsujiguchi and H. Doi: Haikibutu Shigen Junkan Gakkaishi 20 (2009) 77-84). Moreover, basic research on subjecting a used LCD panel to leaching by means of an inorganic acid and then adsorbing and separating indium by means of a reagent impregnated resin (prepared by allowing an ion exchange resin to be impregnated with a reagent for solvent extraction) using the leachate as an object has been conducted (NPD 3: K. Inoue, M. Nishiura, H. Kawakita, K. Ohto, and H. Harada: Kagaku Kogaku Ronbunshu 34 (2008) 282-286).
In these conventional methods, there remain issues relating to efficient indium recovery from a dilute solution containing negligible amounts of indium, the economic efficiency in an indium recovery process, the necessity of the wastewater treatment in a downstream step thereof and the like.
Under such circumstances, it is thought that biosorption (organic adsorption) utilizing an inexpensive bio-material such as a microorganism as an adsorbing agent, as a method for removing a metal from a dilute solution, becomes a promising approach as a method for recovering indium from a used LCD. In biosorption, a phospholipid and lipopolysaccharides that exist on the cell surface layer of a microorganism (a carboxyl group, a phosphate group and the like as functional groups) act as ion exchangers and metal ions are removed from the liquid phase.
For the purpose of treating wastewater containing harmful metals and purifying an aquatic environment polluted with heavy metals, numerous research findings relating to the adsorption and separation of various harmful metals have been reported up to the present time (NPD 4: S. Schiewer and B. Volesky: Biosorption Processes for Heavy Metal Removal, (Environmental Microbe-Metal Interactions, D. R. Lovley (ed.), ASM Press, Washington, D.C., 2000) pp. 329-362). Moreover, biosorption utilizing various microorganisms, as a method for separating and recovering a noble metal from a dilute solution, has been attracting attention (NPD 5: N. Das: Hydrometallurgy, 103 (2010) 180-189, NPD 6: J. Cui and L. Zhang: J. Hazard. Mater. 158 (2008) 228-256).
With regard to biosorption of indium that is a rare metal, PTD 1 (Japanese Patent Laying-Open No. 2011-26701) discloses a method for recovering indium, gallium and tin by treatment with an iron-reducing bacterium from a metal-containing material containing indium, gallium and tin. However, in particular, there is no disclosure about a method for selectively recovering indium.
Moreover, PTD 2 (Japanese Patent No. 4843491) discloses a method for recovering indium by means of an anion exchange resin from an acid solution composed mainly of hydrochloric acid containing indium. However, the anion exchange resin has no selectivity for indium because the resin adsorbs all anions, including a chloride ion derived from hydrochloric acid in an FPD leachate. Moreover, although a hydrochloric acid solution with a high concentration (3.5%:1.2 M) is used as a leaching agent and the leaching is performed over a long period of 60 minutes, for example, under relatively moderate conditions (80° C., atmospheric pressure) in Example 1, the recovery percentage is as low as about 55% (see FIG. 9). Moreover, since the pH of the leachate is less than or equal to 1, there is also a problem that tin dissolves in the leachate and a separating step for tin is required.